1. Field of the Invention
This invention relates to a method and arrangement of providing real time process control by means of multiple communication networks. More particularly, this invention relates to such a process control arrangement which achieves extremely rapid response times for the processes being controlled by utilizing multiple, independent communications networks dedicated to the requirements of the particular data with which they are associated.
2. Description of the Prior Art
In the field of process control systems, recent developments in industrial automation have evolved from those in which a single supervisory computer controlled the process to those using a distributed system of dedicated microprocessors each responsible for a small aspect of the operation of the total control system. As a result of this reliance on a more distributed approach to process control, there has arisen a commensurate need to develop and refine communication schemes between the plurality of processors to ensure that the process would be completed or executed in a timely manner. One example of such a distributed process control configuration is the Westinghouse WDPF.TM. system which has been applied to a wide range of industrial process control operations and is discussed in European Patent Application Nos. EP 0 132 069 and 0 130 802 which are assigned to the same assignee as the present application. In an application of this type of system to an industrial process control configuration, response times of 0.1 to 1.0 seconds could be adequately provided. However, as with other industrial process control configurations, tightly coupled control systems requiring response times of 25 milliseconds or less could not be adequately provided for.
In a process control system where it is required to provide for such a relatively rapid response time, there generally exists a wide diversity in the types of data that are being processed and, in the usage of that data. This wide diversity consequently results in a broad spectrum of data transfer rates. Typically, a single communication network can address very high data rates for limited amounts of data or larger amounts of data for proportionally reduced data rates.
An example of a process control system that would require such a wide diversity in the timing constraints for the different types of data would be a rolling mill process control system for a steel rolling operation. In such a system, it can be appreciated that as a work piece, which in this example is a steel slab, is transported from one roll stand to another, the process of rolling that steel slab into the exact gauge of steel desired, requires precise coordination between the speed and positioning information of the respective roller assemblies at these roll stands. In addition to the rapid coordination of the speed and positioning information, the overall rolling mill operation deals as well, with information for which the timing is not as critical; an example of such information is the historical data which provides the records of how the process has operated from an overall system viewpoint.
Recognizing this need for the rapid handling of certain information, one example of a prior art process control system that could be applicable, utilizes a communications scheme whereby the initialization of the receiving and transmitting stations is accomplished by means of a connecting link rather than a master control unit, such example being found in U.S. Pat. No. 4,177,450 which issued to P. Sarrand on Dec. 4, 1979. In this patent, the described communication arrangement provides that activity on the connecting link is monitored by each station and, based on a predetermined timing scheme, an initiation signal can be transmitted when there has been a lack of activity on the link. Additionally, this patent discloses that the predetermined timing scheme allows the setting of different take over times for the different stations depending on the priority of the various stations. Though this approach does provide for the rapid handling of some data communications between various stations, the effort necessary to establish the timing scheme would appear to be cumbersome in addition to the fact that the flexibility one might desire for the purpose of modifying the overall station arrangement, is greatly reduced. In addition, this approach only provides for one type of communications handling scheme and does not provide for the recognition that handling different categories of data in different manners so that a more precise handling of data for certain operations can be done.